1. Field of the Invention
This invention relates to a method and aqueous suspension for manufacturing a carbonate-containing mechanical pulp and paper at neutral or mildly alkaline conditions. More particularly, it relates to a method and suspension for preventing alkaline darkening of calcium carbonate containing mechanical pulps.
2. Description of Prior Art
Wood-free fine paper is mainly made from bleached chemical pulp and may contain as much as 20% or more of calcium carbonate filler. Producing wood-free fine paper with ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) filler yields significant advantages, both economical and in quality, over acid paper made with clay fillers. Converting from acid to alkaline papermaking enables the use of bright, low cost PCC since carbonate fillers are known to decompose in acid.
Today, many LWC (light weight-coated) and magazine paper mills are running under slightly alkaline conditions, pH 7.2–8.0. The LWC base sheet and magazine paper are made from bleached mechanical pulp and up to 50% chemical pulp, whereas some magazine papers are made from 100% recycled papers which contain up to 60% chemical pulp. The calcium carbonate filler comes from coated broke, from the recycled papers or is purposely added to the stock. The calcium carbonate enhances paper opacity and brightness, improves the printing properties of the paper, and reduces fiber cost.
Despite its many benefits CaCO3 is not used in the production of regular newsprint and mechanical pulp printing grades for two reasons: pulp darkening, and the solubility of CaCO3 in acidic media. If a solution to these drawbacks was found, high quality paper could be made from high-yield mechanical pulp stock, the opacity and brightness could be enhanced using PCC and, the cost of production could be substantially reduced.
A mechanical pulp has an acidic nature because it contains carboxylic and other acidic groups. Mechanical pulps lose brightness when exposed to alkaline environment. Since the pH of CaCO3 suspension is between 8.2 and 10, the pulp will darken in its presence.
As a wet end filler, calcium carbonate is often added into the machine chest of the papermaking system. About 50% of the added calcium carbonate ends up in the whitewater and circulates in the paper machine system. In an integrated paper mill, this carbonate-containing whitewater circulates back to the pulping plant, thus increasing the pH in the pulping process (see FIG. 4). The temperature in mechanical pulping is usually much higher than in a papermaking system and can reach 150° C. in a refiner. A combination of alkaline pH and high temperature is known to have a more negative effect on pulp brightness.
Paper cannot be produced in an acidic medium using CaCO3 as filler because CaCO3 is soluble in such a medium, forming calcium ions and carbon dioxide gas. The lower the pH of the medium, the more [Ca+2] ions dissolve into solution. The solubility of calcium carbonate is also influenced by the level of electrolytes and dissolved acidic wood substances present in solution [Weigl, J. Papermaking problems with systems containing calcium carbonate at faintly acid or basic pH ranges. Wochbl. Papierfabr. 110: 857–866 (1982)].
In U.S. Pat. Nos. 5,043,017 and 5,505,819 methods are disclosed for using special combinations of acids to reduce the pH of calcium carbonate containing pulp to neutral without significantly dissolving the CaCO3. Under neutral pH conditions the darkening of mechanical pulp is reduced. However, when used in the production of groundwood paper at neutral pH conditions (pH 6.5–7.5) a 1 to 5 point brightness loss is often experienced.
A method for reducing alkaline darkening of mechanical pulp due to the presence of a CaCO3 filler is therefore necessary to achieve a successful conversion from acid to neutral/alkaline papermaking.
U.S. Pat. No. 2,173,167 teaches making high-brightness groundwood-containing paper by the addition of calcium carbonate filler. In the method pulp brightness loss caused by alkaline darkening was compensated by the addition of at least (10%) of calcium carbonate filler. In other words the effect of alkaline darkening was masked by increasing the level of calcium carbonate in the sheet. This method is not suitable for grades, such as newsprint, that employ only a few percent of carbonate. Even for papers that contain enough carbonate to overcome the pulp brightness loss, it would be desirable to minimize pulp darkening.
The use of some reducing agents is suggested to decrease light-induced brightness reversion of mechanical pulps. U.S. Pat. No. 5,080,754, for instance, teaches that the reduction of mechanical pulp yellowing can be achieved by using certain formyl compounds. Although in this patent it was also claimed that the reduction of the yellowing could be enhanced by adding calcium carbonate filler, the CaCO3 was applied onto the paper surface, but not to the pulp stock. A method is disclosed in U.S. Pat. No. 5,181,988 for preventing light-induced paper yellowing using hypophosphorous acid and its salts. In U.S. Pat. No. 5,368,689, the use of reducing agents with specifically defined organic acid or its salt. In all these references, costly reducing agents are used to decrease brightness reversion caused by light exposure. These methods might reduce pulp darkening caused by the addition of calcium carbonate filler, but are too expensive.
Sodium sulphite is a cheap product commonly used in the production of: sulphite pulp, chemi-mechanical pulp and chemi-thermomechanical pulp. The sulphite is used to soften lignin in the wood fibre, to reduce refining energy, increase the long fibre fraction and to improve pulp strength. Although it is known that sodium sulphite applied in this manner leads to a brighter pulp; it is seldom used as a bleaching agent because of its low bleaching efficiency. Sodium sulphite is rapidly consumed at refiner or grinder temperatures and would not be useful in decreasing mechanical pulp darkening during subsequent paper making with calcium carbonate filler. Japanese patent J6 3190095 discloses a method using high yield pulp and calcium carbonate to produce lightweight coating base paper. As a part of a high yield pulping process, sodium sulphite is used in the cooking stage at a high dosage, over 8%, at a high temperature, 105–180° C., and at a neutral pH, before the subsequent refining. The purpose of sodium sulphite is to soften the wood chips and partially remove lignin. The sodium sulphite is consumed in the cooking stage and so it is not available to prevent alkaline darkening during papermaking with CaCO3.
U.S. Pat. Nos. 2,178,606, 2,186,040 and 2,242,087 disclose the use of sulphurous acid in the production of filler from dolomitic limestone. The purpose of using sulphurous acid or acid calcium bisulphite is to dissolve the carbonate to remove the magnesium or regenerate the solid particles, or to dissolve portions of the filler particle to make the filler more opaque. Calcium and magnesium sulphites may also be precipitated out and become a part of the produced filler. This patent also claims that the presence of calcium and magnesium sulphite filler in paper produced from mechanical pulp tends to prevent light induced yellowing due to aging and exposure to sunlight.
U.S. Pat. No. 4,183,146 discloses a process for simultaneously drying mechanical pulp and improving its strength and brightness properties. In this process, sodium sulphite is added to a pulp having a solid content of 20 to 50%. The sulphonation and bleaching reactions take place at high temperatures and no mention is made of CaCO3 being present as filler.
In summary, no prior teaching is known that addresses the reduction of alkaline darkening of mechanical pulp due to the presence of calcium carbonate filler during pulp or paper production.